Nautical measured mile markers

When neither SOG data or any other reliable means of estimating Speed Through the Water (STW) is available, Nautical measured mile markers can be used to help calibrate Log speed. Nautical measured mile markers are identified by two pairs of posts or towers. The distance between each pair of markers is 1 nautical mile.

CalibrationSpeedSpeed calibrationMeasured mileMile markersEach marker in a pair is separated by distance and elevation from its partner. The front marker is closer to the water and shorter than the marker behind it.

Figure 1. Markers — side view

  1. Front marker

  2. Rear marker

When the 2 markers appear vertically aligned your vessel is on the correct range line to begin a measured mile run.

Figure 2. Marker alignment (viewed from vessel)

  1. Vessel left of range line

  2. Vessel on range line

  3. Vessel right of range line

CalibrationSpeedThe vessel should already be at top speed and as the first pair of markers appear aligned a stopwatch should be started, when the vessel passes the second pair of aligned markers the stopwatch is stopped.

Figure 3. Measuring a nautical mile

  1. Starting point (start stopwatch)

  2. Measured mile

  3. End point (stop stopwatch)

  4. First pair of markers

  5. Second pair of markers

To provide a more accurate reading the vessel should make between 4 to 6 runs in both directions to allow for tide and wind conditions. The average of the time taken over all runs should be used to calculate Log Speed.

The Log speed can then be worked out by taking the distance travelled (1 nautical mile) and dividing it by the average time taken to perform the run . The resulting calculation is your average speed in knots.